π-conjugated organic molecules (cyclic and/or linear) are widely used in the field of photonics, organic electronics including photovoltaics, optoelectronics and biology due to their geometrical and electronic structures which give them unique properties. The optic and electronic properties of those π-conjugated organic molecules are linked to a parameter named HOMO-LUMO gap (Eg) which corresponds to the energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). This gap should be as low as possible for the above-mentioned applications.
The molecules presenting a low Eg gap are often molecules of high dimensions. However, those molecules are weakly soluble which could be a huge limitation for the above mentioned applications. It is thus important to find new π-conjugated molecules that could present a low Eg gap and which could be soluble in organic medium and water.
Photodynamic therapy (PDT) is a form of phototherapy using nontoxic light-sensitive compounds that are exposed selectively to light, whereupon they becomes toxic to targeted malignant and other diseased cells, and thus can be used in the treatment of cancer for example. Most modern PDT applications involve three key components: a photosensitizer, a light source and oxygen. The wavelength of the light source needs to be appropriate for exciting the photosensitizer to produce reactive oxygen species. The combination of these three components leads to the chemical destruction of any tissues which have either selectively taken up the photosensitizer or have been locally exposed to light. This method has some disadvantages since only cancers accessible to light can be treated such as cancers near the surface of the skin (for example red light only has a penetration of about 1 cm in living tissues). In order to treat other cancers which require a higher penetration into the tissues, the photosensitizer used should absorb in the near infrared region (NIR region) as those radiations penetrate more deeply in the skin.
Regarding photovoltaic technology, about 50% of the solar energy is in the near infrared region. Thus one of the major limiting factors for organic solar cells is the gap between the spectral absorption of the active layer and the solar emitting spectrum. The organic devices actually use various molecules in order to absorb the major part of the solar energy (from visible to near infrared). The use of a unique organic molecule absorbing on a large scale would enable an improvement in yield and costs. Organic solar cells are based on a blend or an electron donor and hole transporting material (such as polythiophene) mixed with an electron acceptor and electronic conductor material (generally fullerene derivatives). Most of the photovoltaic cells actually used are based on silicon. However, this material is expensive and difficult to recycle. Dye-photosensitized solar cells, also called Grätzel (or Graetzel) cells, have been developed based on sensitized inorganic/organic hybride semi-conductors instead of silicon. In Grätzel cell, upon photo absorption, the dye injects an electron in the conduction band of the semi-conductor. These cells especially implement less expensive material, simple and low cost production techniques. Modules obtained are semi-flexible and semi-transparent which also open larger fields of application. There is thus a need to develop dyes for this type of solar cells which can have an improved photoconversion efficiency.
Among the π-conjugated molecules, porphyrins are probably the most important and adaptable macrocycles. The major research activity on porphyrins covers a broad area ranging from chemistry, materials science, physics, biology, engineering and medicine. Porphyrins are highly conjugated heterocyclic macrocycles composed of four pyrrol subunits interconnected via one-atom bridges forming a 16-membered central ring. These porphyrins present 18 delocalized π-electrons. However, these porphyrins do not absorb in the NIR region.
Also known from Muranaka et al (JACS, 2012, 134, 190-193) are analogues of hemiporphyrazine which absorb in the NIR region. However, these compounds are not versatile since their modifications are very limited. Indeed, it should be very useful to modify the macrocyclic molecule in order to modulate the Eg gap, the control the solubility, the geometry . . . .
As a consequence there is a need to provide new macrocyclic molecules having small dimensions, low Eg gap, absorptions in the NIR region, and for which properties can be easily tailored upon chemical modifications (i.e. which are versatile).